The present invention relates to continuous ink jet printing and, more particularly, to a startup sequence for transitioning directionality of the fluid droplets from a state of no charge potential to a state of full charge potential.
Ink jet printing systems are known in which a printhead defines one or more rows of orifices which receive an electrically conductive recording fluid from a pressurized fluid supply manifold and eject the fluid in rows of parallel streams. Printers using such printheads accomplish graphic reproduction by selectively charging and deflecting the drops in each of the streams and depositing at least some of the drops on a print receiving medium, while others of the drops strike a drop catcher device.
During the automatic startup sequence of a continuous ink jet printhead, the ink jets under pressure are stimulated to form uniform droplets that fall past the charge plate and catcher, but are caught in the sealing area of the eyelid seal and catch pan assembly and then are ingested into the catcher throat and returned to the fluid system by vacuum. In one of the many steps that define the startup sequence for continuous inkjet printers, the formed droplets suddenly have a charge potential applied when the printer is started, such that the directionality of the droplets are changed from hitting at the eyelid seal and catcher throat interface and pulled by vacuum into the catcher throat for return to the fluid system, to being deflected upon the face of the catcher for vacuum return to the same.
Prior startup sequences have used the xe2x80x9cbestxe2x80x9d print window voltage, minus a fixed number, to establish the first charge potential to change the droplet directionality. The charge potential was then subsequently stepped until the xe2x80x9cbestxe2x80x9d print voltage was reached. In prior art systems, the purpose of this lowered first charge potential relative to the xe2x80x9cbestxe2x80x9d print voltage was to minimize possible voltage overshoot that might over-deflect the drops and cause a charge plate short problem. In such prior art systems, it is necessary to have the first charge potential only a small amount below the best print voltage to eliminate voltage overshoot induced charge plate shorting.
In recent long array, high resolution printheads such as the VersaMark printheads, manufactured and sold by Scitex Digital Printing, Inc., in Dayton, Ohio, it has been found that it is desirable to lower the first charge potential a greater amount relative to the xe2x80x9cbestxe2x80x9d print voltage. With a fixed voltage difference between the xe2x80x9cbestxe2x80x9d print voltage and the first charge potential, it has been found that while many printheads can startup properly, certain printheads have high failure rates at startup. If the voltage difference is not large enough, the drop deflection overshoot for some printheads can produce charge plate shorts. Conversely if the voltage difference is too large, the first charge potential for some printheads may not change the deflection path enough to cause all of the droplets to hit on the catcher face. The droplets that do not hit upon the catcher face have the adverse ability to cause splatter on the charging electrodes that could interfere with the print droplets or the charge short detection circuit leading to a charge short, flow up the eyelid seal where a path to ground can be formed with the orifice plate leading to a charge short, or wick out of the eyelid seal leading to dripping. As the flow rate and pressure for printheads increases, the severity of the above mentioned problems can potentially lead to poor startup reliability, unless all of the droplets are deflected to hit the catcher resulting in a smoother fluidic transition.
It would be desirable, therefore, to be able to transition directionality of all of the fluid droplets, using the xe2x80x9clowest all catchxe2x80x9d voltage, where the lowest all catch voltage is defined as the lowest charge potential where all of the droplets hit the catcher face.
This need is met by the startup sequence according to the present invention, wherein a sequence in the startup cycle of the printhead deflects ink droplets into catch using a predetermined lowest all catch voltage. Since the lowest all catch voltage is determined during the manufacture of the printhead and can be stored in the printhead memory chip, the method of the present invention improves the startup reliability and eliminates the potential for the above-mentioned problems.
In accordance with one aspect of the present invention, the use of the lowest all catch voltage for deflecting the charged droplets onto the catcher surface enhances the reliability of the startup sequence, particularly as printheads are developed having higher speed and flow rate of droplets. By going from a state where all of the droplets are hitting in the eyelid seal and catch pan assembly to a state where all of the droplets are deflected and caught on the catcher face will help eliminate the possibility of splatter on the charge plate electrodes and/or the charge short detect level circuitry, ink on top of the eyelid seal, and wicking of ink out of the eyelid.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.